Research Of Regulation Effect Of Nitrogen Fertilizer At Heading Stage On Yield And Quality Of Japonica Rice Under High Temperature During Filling Stage

Rice is one of main grain crops in China,which plays a decisive role in the national food security system.Filling stage is the key period to determine the yield and quality of rice,and usually encounters high temperature in the middle and lower reaches of the Yangtze River Valley.Under the trend of global warming,the negative effects of high temperature on rice yield and quality are more and more non-negligible.However,the cultivation regulation measures and mechanisms to mitigate the effects of high temperature on rice are rarely reported.Therefore,Wuyunjing 31 was selected as the experimental material in this study to carry out experimental research on the experimental farm of Wenhuilu Campus of Yangzhou University.The artificial climate chamber was used to simulate the high temperature scene during filling stage,and different amounts and types of nitrogen were applied at heading stage.In 2020,four treatments were set up:natural temperature(NT),high temperature(HT),high temperature+low amount of nitrogen fertilizer(HT+LBN)and high temperature+high amount of nitrogen fertilizer(HT+HBN).In 2022,the treatment of HT+LBN was optimally adjusted to high temperature+foliar nitrogen fertilizer(HT+FN).The difference of nitrogen fertilizer input in each treatment was reflected in the nitrogen dosage at heading stage.By determining rice yield,quality,physicochemical characteristics of grain and physiological and ecological indexes of rice at filling stage,the response of rice yield and quality to high temperature at filling stage was clarified,and the regulation effect and mechanism of nitrogen fertilizer at heading stage on yield and quality of japonica rice under high temperature during filling stage was explored,which could provide theoretical reference for japonica rice cultivation with strong stress resistance,good quality and high yield in the middle and lower reaches of Yangtze River Valley.The main research results are as follows:1.High temperature during filling stage reduced rice yield and lowered rice quality,and affected physicochemical properties of starch.In terms of yield,high temperature during filling stage increased plant temperature,reduced net photosynthetic rate,stomatal conductivity and intercellular CO2 concentration of flag leaves,inhibited photosynthesis of flag leaves,and decreased apparent NSC transport rate of stem sheath,which resulted in insufficient grain filling,increased number of empty grains,decreased 1000-grain weight,and finally significantly decreased yield.In terms of rice quality,high temperature during filling stage deteriorated the processing quality and appearance quality of rice,reduced the brown rice rate,milled rice rate and head rice rate,increased the chalky rate,chalky area and chalky degree,reduced amylose,Fe and Mg contents,and increased the contents of crude protein,albumin and Zn and the enthalpy of starch retrogradation.In terms of physicochemical properties of starch,high temperature during filling stage increased the solubility,relative crystallinity and proportion of large starch granules,but decreased the swelling potential and proportion of small and medium starch granules.2.Nitrogen application at heading stage alleviated the negative effects of high temperature during filling stage on 1000-grain weight of rice to a certain extent,and HT+HBN had the best effect.By increasing transpiration rate of flag leaf,reducing plant temperature,promoting photosynthesis of flag leaf and stem sheath NSC transport to panicle,the filled grain rate and 1000-grain weight of rice were significantly improved,thus increasing the yield.HT+LBN and HT+FN had poor effect on reducing plant temperature,and HT+LBN had little effect on improving photosynthetic characteristics of flag leaves,but it significantly increased Fv/Fo and Fv/Fm.HT+FN had better influence on increasing stomatal conductivity,transpiration rate and qN,but had little effect on net photosynthetic rate.3.Nitrogen application at heading stage improved rice quality under high temperature during filling stage.Compared with HT,HT+HBN significantly improved the processing quality indexes of rice under high temperature during filling stage,and some of the indexes even reached the level of NT treatment.HT+LBN treatment significantly increased the milled rice rate and the head rice rate,but the increase rate was smaller than that of HT+HBN treatment,while there was no significant difference between HT+FN treatment and HT treatment in rice processing quality.In terms of appearance quality,nitrogen application at heading stage significantly reduced the formation of chalkiness,and the chalkiness decreased with the increase of nitrogen application at heading stage in 2020,while there was no significant difference in the chalkiness area of HT+HBN and HT+FN in 2022,suggesting that foliar nitrogen application at heading stage was beneficial to the improvement of rice chalkiness.In terms of cooking and eating quality as well as nutrition quality,nitrogen application at heading stage decreased amylose content,starch retrogradation enthalpy and retrogradation degree,improved starch retrogradation characteristics,and reduced Zn content.Nitrogen application at heading stage increased the protein content,and nitrogen application caused the enthalpy of gelatinization to increase,rice needed to absorb more heat during cooking process,but the enthalpy of gelatinization was decreased by foliar nitrogen application at heading stage.In addition,both HT+LBN and HT+HBN increased the content of prolamin,but there was no significant difference in prolamin content between HT+FN and HT.Only HT+HBN increased Fe content,and no significant difference was found between other nitrogen application treatments and HT treatment.